1. Field of the Invention
The present invention relates to direct sequence spread spectrum communications (DSSS) systems. More specifically, the present invention relates to a single spreader architecture for a DSSS system that can spread the spectrum of a baseband data signal by either on-off keying (OOK), binary phase-shift keying (BPSK), or quadrature phase-shift-keying (QPSK) modulation, depending on the levels of the spreader control lines. More specifically, the present invention relates to a single spreader architecture as described above for use in a code-division multiple-access (CDMA) communication system.
2. Related Art and General Background
Spread spectrum communication techniques offer robustness to noise, low transmission power, and a low probability of intercept. For such reasons, much of the early development of spread spectrum technology was performed by military researchers. Recently, however, the advantages of this technology have led to its increasing use for consumer applications as well: most notably, in advanced digital cellular telephone systems.
Whereas most other communication techniques modulate a carrier signal with one or more data signals alone, spread spectrum techniques also modulate the carrier with a pseudorandom noise or xe2x80x98pseudonoisexe2x80x99 (PN) signal. In the frequency-hopping variant of spread spectrum systems, the value of the PN signal at a particular instant determines the frequency of the transmitted signal, and thus the spectrum of the signal is spread. In the direct sequence spread spectrum (DSSS) variant, the bit rate of the PN signal (called the xe2x80x98chip ratexe2x80x99) is chosen to be higher than the bit rate of the information signal, such that when the carrier is modulated by both signals, its spectrum is spread.
Communication systems that support multiple individual signals over a single channel must employ some technique to make the various signals distinguishable at the receiver. In time-division multiple-access (TDMA) systems, the individual signals are time-compressed and transmitted in nonoverlapping intervals such that they are orthogonal (and thus separable) in time space. In frequency-division multiple-access (FDMA) systems, the signals are bandlimited and transmitted in nonoverlapping subchannels such that they are orthogonal in frequency space. In code-division multiple-access (CDMA) systems, the signals are spread through modulation by orthogonal code sequences such that they are orthogonal in code space and may be transmitted across the same channel at the same time while remaining distinguishable from each other at the receiver.
In a CDMA DSSS system, then, each individual signal is modulated by a data signal and a pseudonoise (PN) signal that is at least nearly orthogonal to the PN signals assigned to all other users, thus spreading the spectrum of the signal while rendering it distinguishable from the other users"" signals. Before spreading and modulation onto the carrier, the data signal typically undergoes various encoding and interleaving operations designed, for example, to increase data redundancy and allow error correction at the receiver. The data signals may also be encrypted to provide extra security against eavesdroppers. The generation of CDMA signals in a spread spectrum communications system is disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention, and the disclosure of which is hereby incorporated by reference.
A demand for higher data rates and a greater variety of service options has recently caused the complexity of CDMA cellular telephone systems to increase. In order to accommodate the various forms of control and data signals required to provide such services, a single system must support multiple forms of spreading modulation from one instant to the next. High-density voice and data transmissions benefit from the use of QPSK spreading, while lower-density traffic can be handled more efficiently using BPSK spreading, and OOK spreading is best suited for certain signalling and control transmissions.
Traditionally, each particular method of spreading to be supported would require a complete and separate spreader circuit. However, this system requirement conflicts with consumer demands for smaller handsets and longer periods of operation between battery recharges, which require reductions in circuit complexity and the number of components used. Also, in order to reduce circuit size and fabrication cost, the spreader should be constructed as much as possible using digital rather than analog (linear) components.
One object of the present invention is to reduce circuit complexity, gate count, and power consumption by using a single spreader architecture that is capable of spreading the spectrum of a baseband data signal by OOK, BPSK, or QPSK modulation, depending on the levels of the spreader control lines.